The proposed research continues to focus on the mechanisms that regulate development of the high affinity antibody responses which are essential to vaccine development. There are several critical aspects to these responses that are entirely B cell specific. First, clonal expansion which occurs in special microanatomic structures called germinal centers (GC). Second, diversification of antibody genes by somatic mutation (SHM) and class switch recombination (CSR), both of which are initiated by activation induced cytidine deaminase (AID). Although AID prefers antibody genes, it is not entirely lg specific and off target activity is the primary cause of B cell cancers in humans. The third B cell specific aspect of high affinity antibody development is selection for clones of B cells that express high affinity receptors. In the first 4 years of the funding period, as part of the original Aims 1 and 2, we examined the mechanisms by which AID targets lg genes and misstargets cancer genes in the germinal center. Moreover, we documented the cellular and cell biological regulation of B cell clonal expansion in the germinal center. We have initiated research on Aim 3 and have made significant progress in understanding how Rif-1 mediates its effects on CSR by studying its interaction partner ZYMD8 which has an important role in AID targeting.